﻿Nervous 
  System 
  o/'Ceratodus 
  Forsteri. 
  181 
  

  

  position 
  and 
  which 
  occur 
  also 
  in 
  Axolotl* 
  \ 
  in 
  these 
  cases 
  

   the 
  fibres, 
  two 
  in 
  number, 
  have 
  each 
  only 
  one 
  axis-cylinder 
  ; 
  

   but 
  as 
  in 
  Ceratodus 
  there 
  are 
  an 
  indefinite 
  number, 
  the 
  homo- 
  

   logy 
  is 
  not 
  complete 
  ; 
  but 
  in 
  Petromyzon 
  there 
  are 
  a 
  large 
  

   number 
  of 
  naked 
  axis-cylinders 
  occupying 
  the 
  ventral 
  columns: 
  

   these, 
  according 
  to 
  Ahlborn 
  f, 
  are 
  divided 
  into 
  three 
  groups, 
  

   two 
  median 
  and 
  one 
  lateral, 
  on 
  each 
  side 
  ; 
  one 
  fibre 
  of 
  the 
  

   median 
  group 
  decussates 
  with 
  another 
  of 
  the 
  opposite 
  side 
  at 
  

   a 
  spot 
  corresponding 
  to 
  where 
  the 
  multiaxial 
  fibre 
  decussates 
  

   and 
  joins 
  the 
  upper 
  acusticus 
  root 
  ; 
  he 
  is 
  not 
  certain 
  about 
  the 
  

   destination 
  of 
  the 
  remainder 
  of 
  the 
  median 
  group, 
  but 
  the 
  

   lateral 
  group 
  passes 
  on 
  without 
  decussating 
  and 
  joins 
  the 
  lower 
  

   acusticus 
  root 
  : 
  thus 
  we 
  have 
  simply 
  to 
  enclose 
  the 
  Miillerian 
  

   fibres 
  in 
  a 
  medullary 
  sheath 
  and 
  we 
  have 
  the 
  multiaxial 
  fibre 
  ; 
  

   the 
  conclusion 
  is 
  therefore 
  obvious 
  that 
  this 
  fibre 
  is 
  the 
  homo- 
  

   logue 
  of 
  the 
  Miillerian 
  fibres 
  of 
  Petromyzon. 
  

  

  The 
  ventral 
  columns 
  of 
  the 
  spinal 
  cord 
  pass 
  forward, 
  and 
  

   spreading 
  out 
  on 
  the 
  ventral 
  edge 
  of 
  the 
  aqueduct 
  of 
  Sylvius, 
  

   are 
  ultimately 
  lost 
  in 
  the 
  walls 
  of 
  the 
  third 
  ventricle 
  or 
  region 
  

   of 
  the 
  thalamus 
  ; 
  anteriorly 
  they 
  are 
  separated 
  from 
  the 
  crura 
  

   cerebri 
  by 
  the 
  ventral 
  transverse 
  commissure. 
  

  

  The 
  crura 
  cerebri 
  appear 
  to 
  go 
  principally 
  into 
  the 
  lateral 
  

   columns 
  or 
  lateral 
  part 
  of 
  the 
  ventral 
  columns 
  ; 
  the 
  outer 
  

   portion 
  of 
  the 
  former 
  furnish 
  the 
  crura 
  cerebelli 
  and 
  also 
  fur- 
  

   ther 
  forward 
  they 
  send 
  fibres 
  into 
  the 
  second 
  layer 
  of 
  the 
  optic 
  

   lobe. 
  

  

  In 
  the 
  spinal 
  cord 
  the 
  only 
  transverse 
  commissure 
  to 
  be 
  

   found 
  runs 
  through 
  the 
  substantia 
  gelatinosa 
  immediately 
  

   above 
  the 
  central 
  canal. 
  

  

  At 
  the 
  posterior 
  end 
  of 
  the 
  medulla 
  oblongata 
  a 
  transverse 
  

   commissure 
  is 
  seen 
  close 
  to 
  the 
  ventral 
  edge 
  ; 
  it 
  first 
  appears 
  

   in 
  the 
  funnel-shaped 
  backward 
  prolongation 
  of 
  the 
  sinus 
  rhom- 
  

   boidalis, 
  and 
  gradually 
  increases 
  both 
  in 
  lateral 
  and 
  vertical 
  

   extent 
  ; 
  more 
  anteriorly 
  it 
  arrives 
  at 
  its 
  greatest 
  development 
  

   at 
  the 
  region 
  of 
  exit 
  of 
  the 
  trigeminus 
  group 
  of 
  nerves, 
  the 
  

   opposite 
  sides 
  of 
  which 
  it 
  serves 
  to 
  connect, 
  as 
  also 
  the 
  region 
  

   of 
  the 
  vagus. 
  During 
  this 
  course 
  it 
  comes 
  to 
  occupy 
  nearly 
  

   the 
  whole 
  of 
  the 
  motor 
  tract. 
  But 
  this 
  is 
  not 
  its 
  termination, 
  

   for 
  it 
  can 
  be 
  traced 
  further 
  forward 
  ; 
  at 
  the 
  anterior 
  end 
  of 
  

   the 
  cerebellum 
  it 
  has 
  become 
  much 
  smaller, 
  but 
  beneath 
  the 
  

   posterior 
  end 
  of 
  the 
  optic 
  lobe 
  it 
  again 
  increases 
  in 
  size 
  and 
  

   appears 
  to 
  contribute 
  to 
  the 
  network 
  found 
  in 
  the 
  problematical 
  

   body 
  (PI. 
  XII. 
  fig. 
  8, 
  d) 
  on 
  the 
  outer 
  side 
  of 
  the 
  optic 
  lobe 
  ; 
  

  

  * 
  Stieda, 
  Zeitschr. 
  f. 
  wiss. 
  Zool. 
  Bd. 
  xxv. 
  

   t 
  L. 
  c. 
  p. 
  263. 
  

  

  